This feature is (in my opinion) very bad when you think a bit longer about it.
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You add a dangerous single point of compromise to your accounts. When your Proton account is compromised, the attacker will have access to all other accounts that you have linked, without resetting the password as they can login with the credentials they just gained.
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I agree with the others here, do NOT have everything in the same basket. I know it is very convenient to have everything in one place but one thing why Google is such a pain is the monopoly and the vendor lock (you can call it ecosystem but lets be real, it is a vendor that locks you into the products). This also comes back to my first point, single point of compromise.
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Yes Proton is build for privacy, even is now switching to be a foundation with solid share holder rules etc. but we do not know where Proton will be in 10 years (it is like with governments, do not give them the tools to control you just because they are nice now, they can change). If Proton would change into a more data collection company/foundation they would have access to many more data points. All webpages/services you linked your proton account with, track pixels etc.
Let’s beat Google by being more respectful to users privacy and not by just copying them, let’s be better!
I bought a Fairphone 3 and put LineageOS on it a few years back and can recommend it. LineageOS is less secure than GrapheneOS as far as I can tell but the privacy aspect is there, as you have a completely degoogled phone. I have some friends that have the same setup on the newer Fairphones and they are also very happy and have a smoother experience than me, because it is a newer phone.
Fairphone is an European country that has a move to open source (https://www.fairphone.com/en/open-source/).
To the compatibility and functionality:
In general you will find an replacement for every app you now use that is from a big company. Open Source came a long way and most alternatives are even better in my opinion.